1. Field of the Invention
The invention pertains to a method for producing steel slabs, where the strand leaving the mold contains molten metal enclosed by a strand shell, and where, in a following strand guide system, the gap width of the guide rolls, which are supported in segments in an upper and a lower frame, can be adjusted continuously in successive steps by adjusting elements which connect the rolls, where
(a) the gap width is changed by oscillation around a centerline of the gap width in such a way that the dynamic influences on the guide rolls are negligibly small;
(b) the amplitude of the gap width oscillations is set to a value which does not cause any plastic deformation of the strand shell;
(c) the current gap width of the guide rolls is detected;
(d) simultaneously, the actuating force of the adjusting elements and the amplitude of the actuating force are determined; and
(e) when the amplitude of the actuating force increases, the gap width is adjusted to a predetermined value and/or is guided as a function of pressure by means of at least one adjusting element.
2. Description of the Related Art
The method described in DE 197 20 768 C1 has the goal of creating a basis on which it is possible to adjust the gap width with precision over the entire strand guide system by the use of simple measuring devices and simple mathematical means. It also has the goal of making it possible to determine reliably the current position of the tip of the crater inside the slab.
The method is based essentially on a dynamic measurement by excitation of the strand in the form of oscillations, as a result of which it can be recognized whether the gap width is tending to become larger or smaller than the optimum gap width at a specific measurement site, so that, if a deviation is detected, suitable measures can be taken to correct it.
When the dynamic measurement is made, the actuating force produces a hysteresis curve relative to the gap width. This curve has a relatively small area when the shell is still thin and the crater relatively large. The hysteresis curve has a relatively large area, however, after the shell has grown and the volume of the crater has decreased. The curve has a very slender form when the strand is completely solidified.
In the known method, the gap width is changed by an oscillation around a predetermined centerline of the desired slab thickness. Characteristic force-distance curves are analyzed, for which purpose the slopes of the characteristic curves in the force-distance plane, their points of intersection, etc., are taken into consideration for the required evaluation.
Although it is stated in the document cited that the selected oscillation value should be chosen so that the dynamic influences on the strand shell, which is still relatively thin after leaving the mold, remain negligibly small and that the amplitude of the oscillating gap width should be set to a value which prevents the plastic deformation of the strand shell, the expert cannot derive any clear, reproducible principle for concrete action from this information. For the known method is based at least implicitly on the fact that the amplitude of the force-distance characteristic on which the mathematical analysis is based is determined within the scope of only a single oscillation period, which means that the measurement distance consists only of the distance covered by a single stroke.
Practical measurements in the situation described above, however, have shown that the stroke would have to be so large that, depending on the starting point of the oscillation, there would be an acute danger of the plastic deformation of the strand shell, deformation which would lead to cracks in the microstructure.